Plastic containers such as bottles of water or any other beverage are manufactured and filled according to different methods including blow molding or stretch-blow molding of a plastic preform.
According to one of these known methods a plastic preform is first manufactured through a molding process and then heated before being positioned inside a mould.
The preform usually takes the form of a cylindrical tube closed at its bottom end and open at its opposite end. The preform is made of plastic, preferably PET.
Once the preform has been positioned within the mould only the open end of the preform is visible from above the mould.
This method makes use of a stretch rod which is downwardly engaged into the open end of the preform so as to abut against the closed bottom end thereof. The stretch rod is further actuated to be urged against the closed end, thereby resulting in stretching the preform.
After the stretching phase has been initiated an incompressible fluid, preferably a liquid, is injected into the preform through its open end. This liquid injection causes expansion of the preform until the preform comes into contact with the inner walls of the mould (defining an inner surface of the mold), thereby achieving the final shape of the container.
In a combined forming-filling container fabrication process, it is important to ensure that the preform is fully expanded into the cavity of the mold and that all of the contours of the resulting container are fully formed.
The injected liquid is preferably the one to be contained in the container as final product. Generally the liquid is a water based product and/or beverage.
When the forming and filling of the container is performed and the container full of its contents, the venting of the container is done. A possible step of adjustment of the volume of the volume of the liquid in the container may be necessary. At this stage of the process the mould is opened for allowing ejection of the container out of the mold and ready for beginning a new production cycle.
The venting of the container to atmospheric pressure, the withdrawal of the stretch rod from the container or the adjustment of the volume of the liquid in the container, which are detailed steps of the above described process, generally results in liquid dropping on the molding and filing apparatus. Some liquid droplets can especially fall on the inner surface of the mould when the container is ejected out of the mould.
The presence of droplets on the inner surface of the mould will affect the appearance of the next container to be produced during the next cycle. Indeed, the residual droplets are an obstacle to the preform's complete expansion onto the mould surface. As a result, creases and ribs are created at the surface of the container to be further produced.
The creases and ribs are unaesthetic and clearly undesirable for products to be sold to consumers.
A possible solution to get rid of these residual droplets could be to blow compressed air within the mould when it is opened for ejection of the container between two molding cycles but this solution increases cycle time and the cost of the process.
Thus, there exists a need for a solution enabling to produce container using an incompressible fluid blow molding or stretch blow molding process in which the containers have flawless appearance.